In the past several years, there has been increasing concern over the health hazard of exposure to radon-222, a radioactive gas produced in the uranium-238 natural decay series. This is in part due to an improved understanding of the radiobiological effects of radon, but more importantly to the recognition of an exposure hazard to the general population. The exposure hazard to uranium miners and mill employees has been recognized for many years, but it has only been recently that radon-222 and its radioactive progeny have been found to pose a potential health hazard in private dwellings. This is in part due to energy conservation measures that lead to nearly airtight structures with little outside air exchange. Radon levels can build up in these closed structures by diffusion from underlying rock and soil through cracks and pores in concrete floors and concrete block foundation walls. Because radon is a gas and its decay products are generally found as suspended particulates, human exposure is primarily through inhalation. Fractions of these radioactive species are retained in the lungs and have the potential of producing lung cancer.
Present monitoring techniques used for radon detection are either of the continuous type which provide activity flux level information on a "real-time" basis or of the integrating type which provide dose information for a selected time period. Present continuous type monitors are generally based on gas proportional or scintillation techniques and are sophisticated instruments more suited to the laboratory than for field or private home applications. The most commonly used integrating type monitors use thermoluminescence detectors (TLD) or solid state nuclear track detectors (SSNTD) which are inexpensive passive devices, but only give integrated dose information for periods of from days to months. Because radon levels can vary an order-of-magnitude over a twenty-four hour period, a home owner needs to have continuous monitoring in order to provide economical heating and air conditioning and at the same time control radon levels. Moreover, a radon source tracking, ventilation control, or remedial action analysis can only be accurately carried out with continuous type detectors. Thus, there is a need for portable, low cost, low power, continuous detection instrumentation for monitoring airborne alpha radiation.
The use of ionization chambers as a means of detecting nuclear radiation is an adaptation of a very old art, going back to nineteenth-century work on conduction in gases. The passage through a gas of alpha radiation of MeV energies produce, through ionization, approximately 10.sup.-14 coulomb of charges before coming to rest. It is upon this effect that the class of detection instruments called ionization chambers are based. The ion pairs produced are collected through the use of an electrostatic field gradient imposed on the ionizing volume. If the field gradient is great enough to result in collection at an electrode before recombination, but insufficient to cause secondary collision ionization, the chamber is said to be operating in the ion or linear region. The linear region generally exists below approximately 100 V/cm. If the field gradient is between 100 and 1000 V/cm, the chamber is said to be operating in the gas proportional region where a controlled "gain" in charge carriers is produced through collision ionization processes. At field gradients greater than approximately 1000 V/cm, the "Geiger" region is reached where every primary ionization results in an avalanche discharge within the chamber.
The Geiger type counter is not suited for airborne radon detection because its sealed-tube design precludes unobstructed sampling of the alpha radiation. Gas proportional type counters are used for radon measurements, but because this type of counter operates with low electron attachment sample gas and high field gradients, it requires costly and high energy consuming power supplies, pumps and sample gas processing equipment for its flow-through chamber.
Ion or electron chamber counters operating in the linear or non-multiplication mode possess the requisite low voltage and current characteristics and are both simple in design and can provide real-time information. They can also be operated in either the current or pulsed mode. However, when operated in the current mode, both the ion and electron chamber suffer from base-line drift and do not discriminate between different radiations. Moreover, current mode ion and electron chambers are highly sensitive to charged particles, such as smoke, and thus require special pre-filtration of sample gases. The electron pulse, sometimes referred to as the fast-pulse mode, can only be applied in low electron attachment gas environments where electrons survive long enough to be collected and counted. This precludes the use of the electron pulse technique in normal air environments due to the high electron attachment coefficient of oxygen and water vapor. The ion pulse or slow-pulse mode is possible in air environments, but because ion mobilities are nominally a thousand times smaller than electron mobilities, the pulses are long (&gt;100 ms), irregularly shaped and poorly suited for electronic counting. Thus, although the first pulse chambers used to detect alpha radiation in air were of the ion pulse type (Greinacher 1927), the fast-pulse type is employed almost exclusively in commercial instruments. Overhoff (U.S. Pat. No. 4,262,203) describes a slow-pulse type alpha monitor, but because the concept is based on a flow-through chamber, relatively large volumes are required for measurements in low level environmental samples. The one liter chamber used by Overhoff requires a high electrostatic potential of 500 volts to maintain field gradients sufficient to overcome ion recombination. Moreover, a multi-staged pulse recognition, shaping and amplification circuit is required to produce countable output pulses. Thus, this approach has the same high power consumption and cost disadvantages as fast-pulse gas proportional counting.